1. Field of the Invention
The present invention relates generally to an exposure system and more particularly to a capacitance gauge tracking apparatus used for an exposure system, a method for tracking a surface of a semiconductor device using the same, a leveling apparatus having a tracking apparatus and a leveling method, and to a semiconductor device made thereby.
2. Description of the Related Art
In a photolithography exposure system used in manufacturing of semiconductor devices, light beams are radiated on a photoresist layer overlying a semiconductor device substrate through a mask having predetermined transparent regions to expose predetermined regions in the photoresist layer. In order to form an accurate predetermined pattern in the photoresist during an exposure process, it is important to constantly control the exposure distance between the photoresist covered semiconductor device substrate and the light source of the exposure apparatus. The semiconductor device substrate should be leveled with respect to the light source by controlling the tilt degree and the position of the substrate mounted on a stage of the exposure system. That is, the focus distance between the substrate and the light source and/or the degree of tilt of the substrate should be controlled.
A prior art apparatus for leveling a substrate by controlling its focus distance or degree of tilt has a sub-unit for tracking the surface of the substrate to measure a distance between the surface of the substrate and the light source, such as an exposure slit. A capacitance gauge tracking apparatus that measures air capacitance (i.e., the capacitance between the probe and the substrate where the probe does not physically contact the substrate surface) may be used for tracking the surface of the semiconductor device substrate. In the capacitance gauge tracking apparatus, a probe tracks the surface of the substrate by being moved over the surface. A gauge measures a distance between the surface of the semiconductor device substrate and an exposure slit based on the signal from the probe. The measured signal from the gauge is fed back to an exposure system controller, such as a leveling apparatus stage controller. The controller positions a stage on which the substrate is mounted based on the measured signal. The photoresist layer on the semiconductor device substrate is then exposed by the light source of the exposure system.
FIG. 1 schematically shows a vertical structure formed on a semiconductor device substrate 10, and FIG. 2 schematically shows the result of tracking the surface of the semiconductor device substrate 10 using a capacitance gauge tracking apparatus.
Referring to FIG. 1, as the level of integration of semiconductor devices becomes higher, a step of a conductive layer 20, such as a storage node, becomes greater. That is, in order to ensure sufficient capacitance to operate a memory cell in a cell array area of the semiconductor device, a thick conductive layer 20, such as a doped polycrystalline silicon layer should be formed. A thick insulating layer 30 is required to overcome the large step of the thick conductive layer 20. However, the insulating layer 30 is relatively thin in a portion 31 covering the upper portion of the conductive layer 20, and relatively thick in a portion 33 burying the gap between the conductive layers 20 or in a portion covering a peripheral circuit area of the semiconductor device where the conductive layer 20 is not formed.
Referring to FIG. 2, when the surface of the semiconductor substrate is tracked by the capacitance gauge tracking apparatus, a measured value output by the apparatus oscillates in accordance with a position of the probe over the surface of the semiconductor substrate 10. The measured value may be a voltage value or a focus value (i.e., the measured voltage value converted by the apparatus to a distance between the substrate and the light source). That is, the focus value oscillates with a period (shown as "P", in FIGS. 1 and 2) corresponding to the period of oscillation in the thickness of the insulating layer 30.
Therefore, a change in the thickness of the insulating layer 30 can have an effect on the substrate position value, such as the focus value, measured by the capacitance gauge tracking apparatus. Although there is no substantial change in the height of the surface of the semiconductor substrate 10 covered by layers such as 20, 30, etc., the value measured by the capacitance gauge tracking apparatus may oscillate instead to indicate a substrate of a varying height of being relatively steady to indicate a substrate of a constant height. Thus, a tracking error is present in the signal output by the tracking apparatus to the stage controller. The signal output by the tracking apparatus erroneously indicates the state of the surface of the semiconductor substrate 10. The tracking error may cause the surface of the semiconductor substrate 10 to be misaligned with respect to the light source, such as the exposure slit of the exposure apparatus, by the stage controller of the leveling apparatus. That is, leveling failure or error may occur due to the tracking error. The leveling failure may greatly reduce a focus margin of the exposure process. The reduction in the focus margin may reduce an exposure process margin to generate an exposure failure which would lead to the eventual failure of the semiconductor device.